The present invention relates generally to radio frequency antennas and, in particular, to broadband compact antennas for use in a communications apparatus.
The increasing demand for multi-channel and broadband applications in wireless communications has necessitated the design of broadband antennas. With the current emphasis on antenna miniaturization, an antenna that possesses a wide impedance bandwidth, a compact structure and a high radiation efficiency is therefore very desirable. However, the design and construction of such an antenna are a significant challenge. Theoretical limitations exist in this endeavor [1, 2]. All references cited herein are listed at the end of this patent document and are incorporated herein by reference.
The designer of practical communications equipment that has wide consumer usage must pay careful attention to the ergonomics of the design of the units, such as the handsets. In general, the consumer requires increasingly more compact equipment that provides more functions. The antenna is a major limitation with respect to its size, its efficiency and its ability to cover a wide frequency range. In the near future, the handsets may be required to cover the cellular bands (800 to 900 MHz), the GPS (Global Positioning Satellite) frequency (1525 MHz), the PCS (Personal Communication System) band (1800 to 2000 MHz) and possibly higher frequency bands. A single antenna that can cover many frequency bands is much more desirable than multiple antennas that cover each frequency band individually. For the above reason, it is an object of the present invention to provide a highly efficient, compact antenna having a wide impedance bandwidth for use in a multi-channel or broadband communications apparatus.
A widely employed antenna on radio handsets and cellular phones is a quarter wave monopole mounted on the radio that uses the radio case (or circuit boards) as the equivalent of a small ground plane to provide a rough equivalent of a dipole antenna. The usage of a monopole on a ground plane and its equivalence to a dipole, is widely known. The fact that a radio case is used instead of a ground plane causes changes in the radiation pattern and input impedance of the antenna, but in most cases acceptable changes or recoverable changes (through minor modification to the monopole) are introduced. Due to the large variations in their input impedances with frequency, thin dipoles and monopoles are considered to be narrowband. Nevertheless, the impedance bandwidth of a dipole or monopole can be substantially broadened by increasing the thickness of the conductor. The well-known cylindrical and bow-tie (triangular) antennas are broadband variants of wire dipole and monopole antennas. A typical bow-tie antenna comprising two triangular sheet of conducting material is depicted in FIG. 1 [3]. To obtain a reduction in antenna size, a triangular sheet of metal can be placed over a ground plane conductor (as shown in FIG. 2) to form a equivalent monopole structure of the bow-tie antenna. Another broadband derivative of the dipole and monopole antennas are sleeve antennas. A sleeve dipole, formed by extending the inner and outer conductors of a coaxial line over a ground plane conductor, is shown in FIG. 3 and it has broadband properties superior to those of a half-wave or full-wave dipole [4]. FIGS. 4-6 [5, 6] illustrate some other varieties of a sleeve antenna.
A slot antenna is a complement of a dipole antenna with dimensions identical to the slot. Because of its low profile, slot antennas have many practical applications in wireless communications, especially where flush installations are needed. According to Babinet""s principle [7], the radiation pattern of a slot antenna in an infinite conducting sheet is the same as that of the complementary dipole antenna, except that the electric and magnetic fields are interchanged. The input impedances of a slot antenna and its complementary dipole are related by
xe2x80x83ZslotZdip=xcex72/4xe2x80x83xe2x80x83(1)
where Zslot and Zdip are the input impedances of the slot antenna and the complementary dipole respectively, and xcex7 is the intrinsic impedance of the surrounding medium (=120xcfx80 in free space). From the relationship as expressed by equation (1), it is evident that the input impedance of the slot antenna is inversely proportional to that of the complementary dipole, or vice versa. FIG. 7 depicts the input impedance of a typical dipole antenna, as a function of frequency, on a Smith Chart. The impedance curve of the complementary slot antenna is also shown in FIG. 8. By examining the two impedance spirals, it can be found that the slot antenna and the complementary dipole always have opposite reactances or susceptances. The dipole resonates, with a practical input impedance of roughly 70 ohms, when its electrical length is an odd number of half wavelengths, whereas the complementary slot is resonant and has a low input resistance when its electrical length is an even number of half wavelengths. Accordingly, if the slot antenna and the dipole are combined in some ways to form a single radiating structure, it is feasible to cause the input reactance or susceptance of the resulting antenna to be cancelled or reduced to some extend over a wide frequency range and to achieve a low input resistance across very wide bandwidths. The present invention thus employs both the slot antenna and the dipole or monopole antenna as the building blocks of the antenna structure and combines them together to achieve a wide impedance bandwidth and physical compactness. The slot antenna and the electric monopole or dipole antenna are connected together in a simple parallel connection or tight magnetic or electric coupling.
It is to be noted that an antenna made of a combined slot dipole and electric monopole connection has been disclosed by Mayes [8-11] using a more complex series and parallel connection of the elemental antennas. Variations of this antenna have been presented by Hall [12-14]. The basic structure of this group of antennas involves a microstrip line with two inputs or outputs mounted under the ground plane [8]. The slot is built into the ground plane to intercept ground currents flowing in the ground plane immediately above the xe2x80x9chotxe2x80x9d conductor of the microstrip line. The slot antenna is therefore connected in series with the microstrip line. The monopole is connected in parallel to the microstrip line at a point coincident with the effective feed point of the slot antenna. The monopole emerges from below the ground plane through the slot. This antenna can produce a cardioid-like radiation pattern when fed on one of the transmission line arms and another cardioid-like radiation pattern oriented in the other direction when fed by the other transmission line arm. Variations of this antenna have been constructed by Hall [12] and by Mayes [11]. Four port extensions of this antenna have been developed by Mayes [11] and by Hall [13]. This antenna can be constructed to have a large bandwidth by terminating one of the transmission lines in a matched resistor [10]. This resistor will increase loss and lower the efficiency of the antenna however. The antenna described in this invention has a more simple connection of the electric monopole and the slot dipole and achieves large bandwidth without an efficiency reducing resistor. The simple connection of the slot and electric antennas permits a great range of electric monopole and dipole elemental antennas to be connected to a range of monopole and dipole slot antennas to provide a wide variety of combined antennas.
In accordance with an aspect of the present invention, a broadband compact antenna comprises an electric dipole or monopole coupled or connected in parallel to a slot antenna. In other aspects, the slot antenna is composed of a flat, square or rectangular conducting sheet with a slot having a variety of possible shapes including a bow-tie or rectangle. The slot is preferably fed at the center by a coaxial transmission line with its outer conductor bonded to the sheet. In another aspect, to obtain broadband characteristics and compactness, a dipole or monopole, formed using either wire, flat strips or shapes formed in sheets of metal, is located in close proximity to the center of the slot. In one embodiment of the present invention, a parasitic dipole is magnetically coupled to a slot antenna by placing a low dielectric spacer between the slot and the dipole. The spacer allows maximum coupling between the slot and the dipole, while preventing a direct electrical contact of the two elements. The parasitic dipole and the slot are oriented so that the polarizations of the two elements are identical. In another embodiment of the present invention, a dipole or monopole is connected to the center of a slot antenna and both antennas are energized by a common coaxial feed. The dipole or monopole is positioned in a plane at an angle or normal to that of the slot antenna. Practical and commercially available shielded (i.e., coaxial) transmission lines have characteristic impedances that cover a relatively small range of values, for example 50 to 75 ohms. Broadband antennas must have an impedance that matches these transmission lines for maximum practical application. Thus, the input impedance of a broadband antenna must be roughly in the range of 50 to 75 ohms. The electrical dipole antenna has input resistances of approximately 70 ohms when its electrical length is an odd number of half wavelengths and has high resistances when its electrical length is an even number of half wavelengths. The slot antenna can be made (by selection of its width) to have input resistances in the 50 to 75 xcexa9 range when its electrical length is an even number of half wavelengths. The input resistances of the slot antenna are high when the slot is an odd number of half wavelengths long. Hence, if the electric dipole and the slot antenna is connected together in parallel, the element with the smaller impedance will dominate and it is practicable to reduce the input impedance of the resulting antenna to a resistance in the 50 to 75 xcexa9 range whenever the two elements are a integral multiple of half wavelength long. At intermediate frequencies, the input reactances or susceptances of the two elements will tend to cancel each other out and the resulting antenna will possess an input impedance of value that is within the practical range. It has been found that, with the above arrangements, a highly efficient, broadband compact antenna, suited for use in hand-portables or other communications equipment, can be achieved by varying the relative dimensions and shapes of the slot and the dipole or monopole. The combination of the slot and dipole or monopole antennas is therefore a more effective radiator than either one alone.